1. Field of the Invention
The present invention relates generally to the mooring of oil transport, production, and drilling vessels in sea ice in the Arctic. More particularly the invention relates to a mooring system which combines a submerged buoyant element structurally connected to an anchor structure on the seabed, and designed to anchor a vessel equipped with a mooring system of the type described in U.S. Pat. Nos. 5,305,703 and 5,477,114.
2. Discussion of the Prior Art
Currently, moorings for vessels in sea ice do not exist. However, a number of proposals have been advanced, using single point moorings of the tower type in which the vessel moors by a structural connection at the deck level of the vessel to a pivoting structure mounted on top of a fixed tower protruding up through the ice. Very large forces need to be transferred in such a device, on the order of 50 to 100 MN. Forces of this magnitude exceed by a large factor the breaking strength of the largest commercially available chain or rope, therefore specially designed very large structural connectors are required.
An alternative means of station keeping that has been proposed is to use high powered, dynamically positioned icebreaking vessels assisted by nuclear or conventionally powered ice breakers. This technology may be feasible for oil shuttle tankers that can tolerate being forced off the mooring, by discontinuing the oil transfer until it can return to the loading point. Such force-offs are, however, much less acceptable for oil production or drilling vessels because in oil shuttle tankers the crude oil can be continually produced into a buffer storage while the shuttle tanker is unavailable, whereas forcing a production vessel off-station causes oil production shut-in, and forcing a drilling vessel off-station with insufficient warning may have catastrophic consequences.